As a method of separating and feeding stacked sheets, such as documents and recording sheets, there is an electrostatic attraction separation method of generating an electric field on an attraction belt and bringing the attraction belt into contact with a sheet to attract and separate the sheet from other sheets.
A background sheet feeder according to the electrostatic attraction separation method includes an attraction separation unit that includes an attraction belt formed by a dielectric member wound around two rollers, a charger for supplying alternating charge to the attraction belt, and a holder holding the attraction belt and the charger. The holder rotatably supports the two rollers, and is fixed to a rotary shaft provided upstream of the rollers in the sheet feeding direction. One end of the rotary shaft is provided with a gear mechanism including a rack and pinion and a swing mechanism serving as a swing device for swinging the attraction separation unit. Further, one of the two rollers disposed on the upstream side in the sheet feeding direction is supported by the holder to be movable in a direction perpendicular to the surface of a sheet bundle.
Prior to the sheet feeding operation, the attraction belt held by the holder via the two rollers is located at a position separated from the sheet bundle. To feed the uppermost sheet of the sheet bundle by separating the sheet from the other sheets, the attraction belt is first rotated and supplied with alternating charge. After the supply of the alternating charge to the attraction belt, the rotation of the attraction belt is stopped. Then, the swing mechanism is driven to swing the attraction separation unit toward the sheet bundle. According to this action, the attraction belt is brought into contact with the uppermost sheet of the sheet bundle to attract the uppermost sheet of the sheet bundle to the attraction belt. After the attraction of the uppermost sheet of the sheet bundle to the attraction belt, the swing mechanism is driven to swing the attraction separation unit in a direction separating from the sheet bundle. According to this action, the uppermost sheet attracted to the attraction belt is lifted by the attraction belt, and is separated from the second sheet. Then, the attraction belt is driven to rotate to feed the uppermost sheet attracted thereto.
In the background sheet feeder, however, the attraction separation unit is configured to be cantilever-supported by the rotary shaft disposed on the upstream side in the sheet feeding direction and serve as a support member. As a result, the downstream end of the attraction separation unit in the sheet feeding direction acts as a free end, and the attraction separation unit vibrates owing to backlash occurring in a gear meshing portion or by resilience of the attraction separation unit. The vibration of the attraction separation unit may cause the uppermost sheet attracted to the attraction belt to separate from the attraction belt. Particularly when the attraction separation unit is swung at relatively high speed to increase productivity, a load due to the inertia of the attraction separation unit is increased. Therefore, it is desirable that the attraction separation unit be reduced in weight. If the attraction separation unit is reduced in weight, however, the above-described vibration becomes more pronounced.
In the background sheet feeder, a rack gear is attached to the rotary shaft serving as a fulcrum of the attraction separation unit. Therefore, a portion for transmitting drive to the attraction separation unit, i.e., a meshing portion between the rack gear and a pinion gear, is located near the rotary shaft serving as the fulcrum of the attraction separation unit. By contrast, the centroid of the attraction separation unit is located on the downstream side in the sheet feeding direction. Therefore, the distance from the gear meshing portion serving as the point of effort in the principle of the lever to the rotation shaft serving as the fulcrum is less than the distance from the centroid of the attraction separation unit serving as the point of load in the principle of the lever to the rotary shaft serving as the fulcrum. As a result, the torque for swinging the attraction separation unit is increased. The increase in the torque leads to an increase in size of a motor for operating the attraction separation unit and the sheet feeder. Further, a relatively large load is imposed on the gear meshing portion, and thus the gears of the swing device are worn away prematurely.